THE EIBS, THE STERNUM, AND THE THOEAX 153 



quadrate cephalic opening or inlet, a very much larger, more circular 

 caudal opening or outlet, and the walls exhibit long, wide slits between 

 the ribs. As each of these intercostal spaces is about four times as 

 wide as the ribs which bound it, only one-fourth of the walls of the 

 thorax may be said to be composed of bone. 



The thorax presents for examination an inlet and an outlet, 

 dorsal, ventral, and lateral walls, and a cavity. 



The inlet or cephalic opening of the thorax (Fig. 97) is bounded 

 ventrally by the manubrium, or first sternebra, and the first pair of 

 costal cartilages ; at the sides, by the shafts of the first pair of ribs ; 

 and dorsally, by the necks and heads of the ribs and the ventral 

 surface of the first thoracic vertebra. Its sides are almost vertical, 

 arching somewhat away from the middle line. Its dorso-ventral 

 diameter is but a trifle greater than the transverse diameter ; its out- 

 line therefore is almost square. Its plane faces toward the head and 

 to a varying degree dorsally. 



The outlet or caudal opening of the thorax (Figs. 98, 103) has the 

 outline of an ellipse the long axis of which is vertical. It is bounded 

 ventrally by the last sternebra and the united costal cartilages of the 

 ribs, and at each side by the overlapping costal cartilages of the eleventh 

 and twelfth ribs, by the ventral end of the twelfth rib and the entire 

 length of the thirteenth rib, the dorsal end of which, with the body 

 of the thirteenth thoracic vertebra, completes the dorsal outline. It 

 is difficult to give accurate measurements to the outlet of the thorax, 

 owing to the changes which it undergoes during life in respiration 

 and in other muscular movements, and after death by the drying and 

 fixation of the caudal ribs and costal cartilages. From the study of 

 transverse sections and plaster casts, the outlet appears to be a third 

 higher than wide. The plane of the opening is not flat, but bent at a 

 sharp angle in its dorsal quarter, since from the sternum to the ends 

 of the thirteenth ribs it is directed dorsally and toward the tail and 

 from the ends of the thirteenth ribs to the vertebra it is directed 

 dorsally and toward the head. 



The dorsal wall of the thorax presents a median longitudinal 

 ridge, formed by the prominent bodies of the thoracic vertebrae. On 

 each side of this ridge is a wide longitudinal furrow, produced by the 

 arching dorsally of the dorsal ends of the ribs. When the animal is 

 standing or walking the dorsal wall is not horizontal, but is directed 



